Field of the Invention
The invention pertains to apparatus and methods for materials processing using microwave energy. More particularly, it pertains to methods for processing photosensitive polyimide (PSPI) films for electronic devices.
Description of Related Art
The use of microwave (MW) energy to enhance the speed of chemical reactions is well known and documented. This unique method of producing heat may cause either the energy of activation (Ea) to be reduced or the kinetics (f and p in Equation 1) of the combination of reactants to increase [see D. A. Lewis, J. D. Summers, T. C. Ward, and J. E. McGrath, “Accelerated Imidization Reactions using Microwave Radiation”, Journal of Polymer Science: Part A; Polymer Chemistry, Vol. 30, 1647-53 (1992)]. It has also been suggested in the chemical literature [see J. Mijovic and J. Wijaya, “Comparative calorimetric Study of Epoxy Cure by Microwave vs. Thermal Cure”, Macromolecules 23:3671 (1990), and J. Mijovic, A. Fishbain, and J. Wijaya, “Mechanistic Modeling of Epoxy-Amine Kinetics: 2—Comparison of Kinetics in Thermal and Microwave Fields”, Macromolecules 25:986 (1992)] that microwave energy would not be practically and commercially feasible but the last two decades has seen the adoption of microwave energy for curing many important resins in a wide range of applications. Some of this contradiction lies in the counter-intuitive nature of the mechanism of microwave heating. Further, even a sound understanding of this mechanism does not anticipate some of the surprises and fortunate opportunities microwave energy offers.k=fpexp[Ea/RT]  Equation 1
The fundamental heating methods of conduction, induction, and convection involve the transfer of heat between one group of more energetic molecules to another group of molecules with less energy by random collision. These collisions are physically sequential and independent of the structure of the molecules other than their bulk enthalpies of heating (ΔH). In contrast, MW irradiation has high penetration depth in many materials of interest (including polymers) which eliminates the necessity of sequential interactions of neighboring molecules in standard heat transfer methods. MW heating solely depends on dielectric relaxation in polarizable bonds which causes dipolar rotation of chemical functional groups. These rotations, at all polarizable bonds whether they are at potential reaction sites or not, create highly efficient and productive motion and collisions between all of the molecules in the irradiation path.
This method of heating would normally not be generally practical for some commercial uses due to the nodes of high and low energy distribution in any electromagnetic field including microwaves. Commercial fixed-frequency, multimode microwave heating systems are well known for spatial non-uniformity in large cavities and for the tendency to initiate arcing and other deleterious effects when metallic materials are processed. However, these effects can be mitigated when necessary through the use of Variable Frequency Microwaves (VFM) as taught, inter alia, in U.S. Pat. Nos. 5,738,915 and 5,879,756, the entire disclosures of which are incorporated herein by reference. VFM has allowed MW curing to become commercially useful in many more industrial applications by creating highly uniform fields without risk of metal arcing.
The manipulation of reaction temperatures using microwaves has been found to be consistently useful in the lowering of the measured reaction (or “cure”) temperature in bulk materials without special chemical modification. Unmodified polyamic acid resins that are fully imidized in conventional ovens above 375° C., can be fully imidized with MW at temperatures as low as 200° C. [see R. Hubbard, Z. Fathi, I. Ahmad, H. Matsutani, T. Hattori, M. Ohe, T. Ueno, C. Schuckert, “Low Temperature Curing of Polyimide Wafer Coatings”, Proceedings of the International Electronics and Manufacturing Technologies, (2004), and R. Hubbard, “Reduced Stress and Improved 2.5D and 3DIC Process Compatibility With Stable Polyimide Dielectrics”, Proceedings of the International Wafer Level Packaging Conference, Nov. 4-7, 2013, San Jose, Calif., for further background information].
The majority of polyimides used as dielectric coatings on wafers in the microelectronics industry have photosensitive properties, which allow them to be directly patterned without the additional steps of photoresist coating, mask exposure, development, and removal [see K. Horie and T. Yamashita, “Photosensitive Polyimides—Fundamentals and Applications”, Lancaster, Pa., Technomic Publishing Co., Inc., pp. 15-18 (1995)]. This useful property is accomplished by the modification of some sites of the polyamic acid (PAA) precursor resin with a photosensitive methacrylate alcohol to form a photosensitive polyamic ester (PAE) as shown in FIG. 1. The alcohol depicted is one or more in the methacrylate family (R—CH2CH2OC(O)CH═C(CH3)2) of monomers and oligomers that is known to crosslink by UV light exposure and forms the basis of many photoresist material families.
Now the PAA/PAE copolymer can be directly photo patterned like a conventional photoresist. As shown schematically in FIG. 1, areas exposed to light through mask openings are crosslinked at the photoactive group “R” creating areas that are less soluble in developer solutions. The more soluble areas are removed by the developer leaving high resolution patterns. The “curing” of photosensitive polyimide films now involves (1) the imidization (or ring closure) reaction step, and (2) the release of the acrylate residue by-product of that imidization as shown schematically in FIG. 2. Removal of the acrylate residue is subsequently achieved at the same 375° C. soak temperature.
The chemistry of the acrylate residuals involves decomposition reactions at temperatures typically in excess of 350° C. for extended times of at least an hour depending on the extent of residue removal required [see M. Zussman and R. Hubbard, “Rapid Cure of Polyimide Coatings for Packaging Applications”, Proceedings of The 13th Symposium on Polymers for Microelectronics, Wilmington, Del., (2008)]. At the lower curing temperature of 350° C. for an hour, it was found as expected that the convection process did not remove substantial amounts of the acrylate residue while the VFM process appeared to remove almost all of the residues. In FIG. 3 the remaining acrylate residue peak is shown in the Dynamic Mechanical Analysis (DMA) of the convection cured sample but not in the VFM cured sample. The Thermal Gravimetric Analysis (TGA) in FIG. 4 displays a 1% weight loss at a much higher temperature for VFM (487° C., top curve) than for convection cure (376° C., bottom curve) which corroborates the DMA conclusions [M. Zussman and R. Hubbard, “Rapid Cure of Polyimide Coatings for Packaging Applications”, Proceedings of The 13th Symposium on Polymers for Microelectronics, Wilmington, Del. (2008)].
It is very important to carry out the conventional high temperature decomposition reactions with a low level of oxygen (<100 ppm), in order to avoid oxidative degradation of the surface of the desired polyimide dielectric film. Decomposition of the polyimide backbone degrades the required electrical properties of the dielectric film as well as producing a brittle dark film.